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Genetic fingerprinting with VNTRs

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Contents

What is a Genetic Fingerprint?

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What is Genetic Fingerprinting Used For?

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Recall Questions

For this topic you require prior knowledge of gel electrophoresis and DNA probes. You can test your knowledge on these below.

What charge does DNA carry and which way does it move in gel electrophoresis?

DNA is negatively charged and moves towards the positive electrode (anode).

What is the purpose of gel electrophoresis in genetic fingerprinting?

To separate DNA fragments based on their mass / size.

What is a DNA probe?

A short, single-stranded DNA sequence that is complementary to a specific DNA region and labelled (e.g. with a radioactive or fluorescent marker) to detect that sequence.

Topic Explainer Video

Check out this @LauraDoesBiology video that explains genetic fingerprinting with VNTRs or read the full notes below. Once you've gone through the whole note, try out the practice questions!

What is a Genetic Fingerprint?

A genetic fingerprint (also known as DNA profiling) is a unique pattern of DNA fragments created by analysing variable number tandem repeats (VNTRs) — short sequences of bases that are repeated in non-coding regions of DNA.

  • The number and length of these repeats varies between individuals, making them ideal for identification.
  • Everyone (except identical twins) has a unique combination of VNTRs, so their genetic fingerprint is unique.

Here, GATA is an example of a VNTR. There is a variable number of this repeated sequence in each of these three samples of DNA, taken from three individuals.

 

Step-by-Step: How to Make a Genetic Fingerprint

1️. Extraction of DNA

  • DNA is extracted from a sample (e.g. blood, saliva, or hair).

2️. Amplification using PCR

  • The DNA is amplified using PCR to produce many copies, focusing on regions containing VNTRs.

3️. Digestion

  • The amplified DNA is cut using restriction endonucleases, which cut at specific base sequences (recognition sites) either side of the VNTRs.

4️. Separation- Gel Electrophoresis

  • DNA fragments are placed in wells of agarose gel and separated by mass and charge using an electric current.
  • Smaller fragments travel further and faster through the gel.

  • The double stranded DNA is then made single stranded by immersing the gel in an alkaline solution to break the hydrogen bonds between base pairs.

5. Transfer to Membrane (Blotting)

  • DNA is transferred from the gel to a nylon membrane by Southern blotting.

6. Hybridisation with Gene Probes

  • The membrane is treated with DNA probes that are:
    • Complementary to VNTR sequences.
    • Radioactively or fluorescently labelled.

7️. Detection

  • The probe binds to complementary DNA fragments (the VNTRs)
  • If radioactive probes are used, an X-ray film is applied to the membrane to visualise the DNA
  • If fluorescently labelled probes have been used, a UV light will be used for visualisation.
  • The resulting pattern of bands is the individual's genetic fingerprint.

What is Genetic Fingerprinting Used For?

Application

Purpose

Forensic science

Matching DNA from a crime scene with suspects

Paternity testing

Determining biological relationships

Medical diagnostics

Identifying genetic mutations linked to inherited diseases

Conservation biology

Studying genetic diversity in endangered species

Evolutionary studies

Comparing DNA between species or populations

Examples of genetic fingerprints

Here you can see half of the DNA fragments in the child’s genetic fingerprint match to DNA fragments in the fathers, the other half of the child’s DNA fragments match to DNA fragments in the mothers.

Here you can see the genetic fingerprints of DNA found at the crime scene match to the victim and suspect 2.

Key Terms

  • Genetic Fingerprinting: A technique to produce a unique pattern of DNA fragments from VNTRs.
  • VNTR (Variable Number Tandem Repeat): Short, repeating sequences in non-coding DNA that vary between individuals.
  • Restriction Endonuclease: An enzyme that cuts DNA at specific base sequences / recognition sites.
  • DNA Probe: A short, single-stranded DNA sequence that is complementary to a specific DNA region and labelled.
  • Gel Electrophoresis: A technique used to separate DNA fragments by mass and charge.
No answer provided.

Exam Tip

You may get a long answer question on this topic.

Chunk the method into the 7 steps described above:

  1. Extraction
  2. Amplification (using PCR)
  3. Digestion (using restriction enzymes)
  4. Separation (using gel electrophoresis)
  5. Southern blotting
  6. Hybridisation (with probes)
  7. Detection
No answer provided.

Describe how genetic fingerprinting may be carried out on a sample of DNA. (6 marks)

  1. DNA is cut using restriction enzymes
  2. Use electrophoresis to separate DNA fragments
  3. Fragments separated by length / mass
  4. Make DNA single-stranded
  5. Southern blotting / transfer DNA to nylon membrane
  6. Apply probes
  7. Radioactive / fluorescent probes should be used
  8. Use X-ray film / UV light to visualise the DNA (7 and 8 should be consistent)
  9. Reference to VNTRs in answer

Practice Question

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!